A KECK HIRES DOPPLER SEARCH FOR PLANETS ORBITING METAL-POOR DWARFS. II. ON THE FREQUENCY OF GIANT PLANETS IN THE METAL-POOR REGIME

Abstract

We present an analysis of three years of precision radial velocity\nmeasurements of 160 metal-poor stars observed with HIRES on the Keck 1\ntelescope. We report on variability and long-term velocity trends for each star\nin our sample. We identify several long-term, low-amplitude radial-velocity\nvariables worthy of follow-up with direct imaging techniques. We place lower\nlimits on the detectable companion mass as a function of orbital period. Our\nsurvey would have detected, with a 99.5% confidence level, over 95% of all\ncompanions on low-eccentricity orbits with velocity semi-amplitude K > 100 m/s,\nor M_p*sin(i) > 3.0 M_JUP*(P/yr)^(1/3), for orbital periods P< 3 yr. None of\nthe stars in our sample exhibits radial-velocity variations compatible with the\npresence of Jovian planets with periods shorter than the survey duration. The\nresulting average frequency of gas giants orbiting metal-poor dwarfs with -2.0\n< [Fe/H] < -0.6 is f_p<0.67% (at the 1-sigma confidence level). We examine the\nimplications of this null result in the context of the observed correlation\nbetween the rate of occurrence of giant planets and the metallicity of their\nmain-sequence solar-type stellar hosts. By combining our dataset with the\nFischer & Valenti (2005) uniform sample, we confirm that the likelihood of a\nstar to harbor a planet more massive than Jupiter within 2 AU is a steeply\nrising function of the host's metallicity. However, the data for stars with\n-1.0 < [Fe/H] < 0.0 are compatible, in a statistical sense, with a constant\noccurrence rate f_p~1%. Our results can usefully inform theoretical studies of\nthe process of giant planet formation across two orders of magnitude in\nmetallicity.\n